The theory that DNA accumulated cross-links during aging is being tested by a study of DNA reassociation kinetics from young and old C57BL/6J male mice. If appreciable cross-links do accumulate, this should be detectible as an increased amount of rapidly reassociating DNA. Transcription from unique DNA sequences is being assayed in liver and brains from aging mice and from the neonatal rodent. We wish to establish whether postnatal development and aging involve selective changes in gene activity, as do earlier developmental stages. Because of the importance of steroid hormones to aging, we are studying the corticosterone receptors in the hippocampus. We have detected significant age-related decreases in a high affinity corticosterone binding protein which is unique to brain.